Beam current control for electron beam machines



NITRO@ XR 391779434 @j April 6, 1965 J. A. HANSEN 3,177,434

BEAM CURRENT CONTROL FOR ELECTRON BEAM MACHINES Filed Aug. 2, 1962 2 Sheets-Sheet 1 AGE/V7' April 6, 1965 J. A. HANSEN 3,177,434

BEAM CURRENT CONTROL FOR ELECTRON BEAM MACHINES Filed Aug. 2, 1962 2 sheets-sheet 2 Q w m N Q l n i 5 xg Q Q i@ l' ulivi "lil:

United States Patent O ice 3,177,434 BEAM CURRENT CONTROL FOR ELECTRON BEAM MACHINES John A. Hansen, Granby, Conn., assigner to United Aircraft Corporation, East Hartford, Conu., a corporation of Delaware Filed Aug. 2, 1962, Ser. No. 214,313 2 Claims. (Cl. 328-267) My invention relates 'to a bias voltage supply for devices which utilize a beam of charged particles. More particularly, my invention relates to controlling the beam current in an electron beam machine by varying the bias or control voltage applied between the control electrode and cathode of such machines.

Electron beam machines, as they are generally known, are devices which use the kinetic energy of an electron beam to work a material. U.S. Patent No. 2,793,281, issued May 21, 1957, to K. H. Steigerwald, discloses such a machine. These machines operate by generating a highly focused beam of electrons. The electron beam is a welding, cutting and machining tool which has practically no mass but has high kinetic energy because of the extremely high velocity imparted to the electrons. Transfer of this kinetic energy to the lattice electrons of the workpiece generates higher lattice vibrations which cause an increase in the temperature within the impingement area suicient to accomplish work.

It is a fact that a beam of highest power densitypower per unit of impingement area-is more effective. That is, a high power density beam can accomplish the required work in the shortest possible time and thus minimize heat conduction to the material adjacent to the area being worked. Of course, the beam power density must be varied in accordance with the type of operation to be performed and the characteristics of the material to be worked. In order to obtain high power density, precise electron opties have to be applied in focusing the beam. Beam power density is defined as:

(l) D-A where i=beam current V=electron accelerating voltage A :area of beam impingement on workpiece From the above discussion and Equation l it becomes apparent that optimum conditions for working material with an electron beam require the smallest possible spot size consistent with the type of electron optics used. Thence. once the beam has been focused for minimum spot size, beam power density may be controlled by varying the beam current.

In electron beam Welders and cutters, once the beam has been focused, the beam current is controlled by means of a variable control voltage applied between a control electrode and the machincs cathode. This bias or control voltage regulates the flow of electrons in the same manner as does the grid voltage in the usual triode type vacuum tube. Since it is the usual practice in this field to have the anode or accelerating electrode at ground potential, the cathode must be maintained at a very high negative voltage in order to accelerate the electrons to the required energy level. In prior art machines, the control voltage is supplied from an independent voltage source and is applied between the highly negative cathode and the control electrode. This necessitates two voltage sources, one for the acceleration voltage and one for the control voltage. When using an independent control voltage source it becomes necessary ,to utilize an isolation transformer between the control voltage supply rectifier,

3,177,434 vPatented Apr. 6, 1965 which is essentially at cathode potential, and the primary source of A.C. power for the machine, which is essentially at ground potential. As is well known, such transformers are expensive and space consuming. Further, use of a separate source of control voltage results in the beam current being very sensitive to fluctuations in the voltage from said primary source of power which in turn cause. corresponding fluctuations in the accelerating and control voltages.

lt is therefore an object of my invention to produce a voltage for controlling a stream of charged particles.

It is another object of my invention to control the current in an intense beam of charged particles.

It is still another object of my invention to provide a simple, less expensive and more reliable beam current control for an electron beam machine.

It is also an object of my invention to provide both the adjustable control voltage and the acceleration voltage for a beam of charged particles from a single voltage source.

It is yet another object of my invention to provide an improved beam current control for an electron beam machine that is relatively insensitive to supply voltage fluctuations.

These and other objects of my invention are accomplished by novel circuitry which utilizes the electron accelerating voltage itself and a self-biasing effect as produced by the electron beam current to generate the control voltage for the beam current control electrode.

My invention may be better understood and its numerous advantages will become apparent to those skilled in the art by reference to the accompanying drawing in which like reference numerals apply to like elements in the various figures and in which:

FIGURE l is a schematic view of a preferred embodiment of my invention.

FIGURE 2 is an apparatus which may be inserted in the circuit of FIGURE 1 to enable it to operate in a pulsed beam mode of operation.

FIGURE 3 is a graph showing two typical triode characteristic curves for an electron beam -machine which clearly illustrate the advantages inherent in my invention and a method for determining beam current when my invention is utilized.

Referring now to FIGURE 1, an electron beam machine is indicated generally at 10. The machine cornprises a vacuum chamber containing a workpiece 12 positioned on a movable table 14, It should be noted that it is possible, by means known in the art, to locate the workpiece 12 outside of the vacuum chamber. The machine also comprises a source of electrons, means for forming the electrons emitted by said source into a beam 11 and a magnetic lens 13 for focusing the beam at the workpiece. The source of electrons comprises a directly' heated cathode 16 which is supplied with heating current from filament current supply 26 and which also has a negative acceleration voltage applied thereto. This negative voltage is derived from high voltage supply 18 which has its positive terminal connected to ground. An aperturcd anode 20 is positioned in the machine between the cathode and the workpiece. The anode is connected to the case of the machine which is grounded. The difference in potential between the cathode 16 and anode 20 causes the electrons emitted from the cathode to be accelerated toward the workpiece 12.

Under operating conditions, the beam impinges on workpiece l2 where it gives off kinetic energy in the form of heat. The workpiece may be moved beneath the beam by movable table 14 and the beam may be detlected over the workpiece by means of deflection coils 15. Positioned adjacent cathode 16 is a control electrode 22. This control electrode may be of the Wehnelt cylinder type such as disclosed in U.S. Patent No. 2,- 771,568, issued November 20, 1956, to K. H. Steigerwald. The control electrode is connected -to the negative terminal of source 18 and is thus maintained at a voltage that is more negative than voltage applied to cathode 16. The magnitude of this bias or voltage difference is variable in the manner to be described below.

Connected across high voltage supply 18 is a voltage divider consisting of rheostat R2 and series resistor R2. The junction between the end terminals of rheostat R2 and resistor R3 is connected via conductor 28 to cathode 16 through resistors R5 and R6 which distribute the beam current evenly across the cathode. The movable wiper arm of rheostat R2 may be similarly connected to the cathode by closing switch S1. `With switch S1 in the open position a portion of the total high voltage, V, from supply 18 will appear across R2 and will thus constitute the control voltage between cathode 16 and control electrode 22. The values of R2 and R3 are so chosen that, with switch S1 in the open position, the resulting bias will cut off the beam current in machine 10. This control voltage is given by the formula:

The position of the wiper arm of rheostat R2 is irrelevant when switch S1 is open and the beam thus cut off. However, if switch S1 is closed, part of R2 between the wiper arm and the junction with R3 becomes essentially short circuited via switch S1, resistor R4 and conductor 28. The control voltage or bias is thus reduced in the same ratio as is the resistance of R2. If the new control voltage is less than that required for cutoff, beam current will flow in machine thus generating an additional voltage across R2 and increasing the bias until av balanced condition exists. Resistor R, which is much smaller in resistance value than R2 prevents the cathode and control electrode from assuming the same potential by xing the lower limit of the resistance for the upper portion of the voltage divider.

Since R3 is much larger in value than R2, the voltage between the machines anode and cathode or the beam accelerating voltage is approximately equal to the voltage from source 18. Hence, the new control voltage may be calculated by the formula:

tra V....=R.' (141%) where:

=beam current R2=the combined resistance of the portion of rheostat R2 not shorted out and R4 For chosen values or R2 and V, a plot of the control voltage as a function of beam current will be a straight line and the point of intersection between this line and the corresponding triode characteristic curve, plotted for the machine, determines the steady state beam current for the chosen values of R2 and V. FIGURE 3 shows a pair of characteristic triode curves, beam current versus control voltage, for an electron beam machine operating with acceleration voltages of 120 kv. and 150 kv. lntersecting these curves are the above discussed straightline plots of control voltage as a function of beam current for two values of V and three values or R2', assuming R2 equals 30 megohms.

FIGURE 3 also illustrates another advantage of my invention. For a fixed value of R2', the beam current 'is much less influenced by uctuations in accelerating voltage than would be the case if an independent control voltage source were utilized, As an example, if V is dropped from 150 kv. to 120 kv. when R2' equals 25K ohms, the beam current, when my invention is used, drops only from 13.7 ma.l to 9.6 ma. With prior art machines, the beam currentvwould drop all the way to 2.4 ma. or, as shown in FIGURE 3, to the value indicated by the characteristic curve for the new value of V.

Referring'again to FIGURE 1, since the cathode 16 operates at a very high negative voltage, it is obvious that means must be provided for closing switch S1 through an isolated shaft or actuator. For this purpose, switch S1 is operated by solenoid 30 which is activated by switch S2. Capacitor C1 which bridges switch S1, functions as an are suppressor when the switch S1 is opened. The wiper arm on rheostat R2 must similarly be remotely controlled. This control is accomplished by means ofl a servomotor 32 the shaft of which is coupled to the wiper arm of R2. The position of the shaft of motor 32 is controlled from the operators console 34 in a manner well known in the art. Reference numeral G1 indicates a spark gap which, in the case of an arc over in the machine between the cathode and anode, will prevent the full voltage from supply 18 from being applied across R2 and R4. Connected in the circuit between the end terminal of resistor R3 and ground may be a kilovoltmeter 24 for monitoring the acceleration voltage V. R3 thus also serves as a series resistor for meter 24 and further as a bleeding resistor to remove static charges from the electron gun and high voltage supply under deenergized conditions. Ammeter 34 may be connected between ground .and the positive terminal of the high voltage supply for monitoring the beam current.

The circuit of FIGURE l will be used when it is desired to employ a steady beam for performing the particular work to be accomplished. In welding or cutting certain materials, it has been found to be desirable to utilize a pulsed electron beam so as not to cause thermal damage to the workpiece. The apparatus of FIGURE 2 may be employed in the.circuit of FIGURE l to permit the pulsed beam mode of operation. When it is desired to work with a pulsed beam, the jumper J of FIGURE 1 is removed and triode 36 of FIGURE 2 is inserted in its place. Switch S1 is then closed. The triode 36 will be biased off by pulse control 38. Therefore, the circuit between the wiper arm of rheostat R2 and cathode 16 of machine 1t) will remain open and the machine will thus be biased off by the potential on conductor 28. When, in response to pulse control 38, triode 36 is biased to its conducting state, the portion of R2 between the wiper arm and the junction with R2 will be essentially short circuited through resistor R4 and machine 10 will be biased vto its conducting state for the reasons discussed above. The apparatus of FIGURE 2 permits a much higher pulse repetition rate than could be realized by opening and closing the switch S1 manually or by mechanical means.

While a preferred embodiment has been shown and described, various modifications and substitutions may be 1 made without deviating from the scope and spirit of my invention. Thus my invention is described by way of illustration rather than limitation and accordingly it is understood that my invention is to be limited only by the appended claims taken in view of the prior art.

I claim:

1. A bias voltage supply for an electron beam welding and cutting machine which has at least an anode, cathode, and control electrode comprising:

a source of direct current voltage,

means electrically connecting the negative terminal of said voltage source to the control electrode of the machine,

a potentiometer having one end terminal electrically connected to the negative terminal of said voltage source,

a resistance element having a first end connected to the other end terminal of said potentiometer so as to be in series therewith,

means connecting the second end of said resistance element and the anode of the machine to the positive terminal of said voltage source,

means connecting the junction between said p otentiometer and resistance element to the cathode of the machine whereby a bias voltage suflicient to prohibit current flow in the machine will appear between the cathode and control electrode, and

means including a remotely controllable normally open switch connected between the variable tap on said potentiometer and said cathode for short-circuting a preselected portion of said potentiometer whereby the negative bias voltage will be reduced by closing said switch and a desired amount of current will ow in the machine.

2. The apparatus of claim 1 further comprising:

electronic switch means connected in series with said mechanical switch means.

References Cited by the Examiner UNITED STATES PATENTS GEORGE N. WESTBY, Primary Examiner.

DAVID J GALVIN, Examiner. 

1. A BIAS VOLTAGE SUPPLY FOR AN ELECTRON BEAM WELDING AND CUTTING MACHINE WHICH HAS AT LEAST AN ANODE, CATHODE, AND CONTROL ELECTRODE COMPRISING: A SOURCE OF DIRECT CURRENT VOLTAGE, MEANS ELECTRICALLY CONNECTING THE NEGATIVE TERMINAL OF SAID VOLTAGE SOURCE TO THE CONTROL ELECTRODE OF THE MACHINE, A POTENTIOMETER HAVING ONE END TERMINAL ELECTRICALLY CONNECTED TO THE NEGATIVE TERMINAL OF SAID VOLTAGE SOURCE, A RESISTANCE ELEMENT HAVING A FIRST END CONNECTED TO THE OTHER END TERMINAL OF SAID POTENTIOMETER SO AS TO BE IN SERIES THEREWITH, MEANS CONNECTING THE SECOND END OF SAID RESISTANCE ELEMENT AND THE ANODE OF THE MACHINE TO THE POSITIVE TERMINAL OF THE VOLTAGE SOURCE, MEANS CONNECTING THE JUNCTION BETWEEN SAID POTENTIOMETER AND RESISTANCE ELEMENT TO THE CATHODE OF THE MACHINE WHEREBY A BIAS VOLTAGE SUFFICIENT TO PROHIBIT CURRENT FLOW IN THE MACHINE WILL APPEAR BETWEEN THE CATHODE AND CONTROL ELECTRODE, AND MEANS INCLUDING A REMOTELY CONTROLLABLE NORMALLY OPEN SWITCH CONNECTED BETWEEN THE VARIABLE TAP ON SAID POTENTIOMETER AND SAID CATHODE FOR SHORT-CIRCUITING A PRESELECTED PORTION OF SAID POTENTIOMETER WHEREBY THE NEGATIVE BIAS VOLTAGE WILL BE REDUCED BY CLOSING SAID SWITCH AND A DESIRED AMOUNT OF CURRENT WILL FLOW IN THE MACHINE. 